Different forms of the double helix: architecture and function

Alexander Rich

Resum

The DNA double helix was discovered 50 years ago. This motif was found in several other places, most notably in RNA molecules. It is also found in DNA-RNA hybrids as well as transfer RNA molecules where double helical segments combine to form a more complex structure. A distinct variant of the DNA molecule was discovered in which the helix turns left-handed, which has led to practical consequences in the treatment of certain viral diseases. Since the molecule readily converted from A to B form, it clearly suggested that there was another form the double helix could adopt in the A form. Thus, the initial mystery ?why are there two such forms? It was known at the time that there are two major nucleic acids, one DNA and the other RNA, in which the sugar phosphate backbone contained ribose rather than the deoxyribose of DNA. One of the early questions that arose immediately after the Watson-Crick formulation was related to the nature of the RNA molecule-could it form a double helix?